Inductors store energy in a magnetic field produced by current through a wire coil and are often used to form radio frequency (RF) circuits. The need to miniaturize electronic circuits applies to RF circuits because of the demand for semiconductor chips used in cellular phones and wireless modem. Although many other digital and analog circuits, such as operational amplifiers and microprocessors, have been successfully implemented in integrated circuits. RF circuits remain, however, a challenge to miniaturization of electronic circuits.
The difficulty of fabricating high quality factor (Q value) inductors in integrated circuits which are suitable for RF applications arises mainly because semiconductor substrates induce energy loss. The low energy consumption is also a concern for inductors employed in devices such as cellular phones and wireless modems.
Because Q value is directly proportional to the inductance (L) and the angular frequency (ω), and inversely proportional to the resistance (R). High Q value designs strive to increase the inductance and decrease the resistance, while keeping parasitic capacitance to a minimum so that high oscillation frequencies can be achieved.
RF circuits demand inductors with high inductance values. However, high inductance values necessitate a large silicon chip area which prevent miniaturization of such circuits. Moreover, the enlarged area of a conductive trace increases the parasitic capacitance which reduces resonant angular frequency of oscillation and Q value.
One way of improving the self resonance frequency and Q value of an inductor is to create an air gap underneath the conductive trace by etching away the semiconductor substrate. This, however, causes the inductor to be effectively suspended without support. Other techniques also have been developed in the semiconductor industry but with limited results. For example, a known technique employs wide metal lines. Unfortunately, because both the inductor area and the parasitic capacitance are increased, the oscillation frequency decreases and, thus, the useful frequency range is limited.
Accordingly, there is a need for further downsizing of induction elements, operating at high RF frequencies with low losses and high efficiency. There is a need in the art for a high Q value inductor that has a high resonance frequency and high inductance, and that occupies a minimal substrate area. In addition, there is a need for a process for fabricating such an inductor.